Frequency discriminator



Feb.` 5, 1963 D. c. DAvls ETAL 3,076,940

FREQUENCY DISCRIMINATOR Filed Jan. 29, 1957 3 Sheets-Sheet 1 Jo//V ,e 570.4 rz j; f6' 5- INVENTORS BY al 'It fraz/Vex D. c. DAvls ETAL 3,076,940 FREQUENCY DIscRIMzNAToR Feb. 5, 1963 Filed Jan. 29, 1957 3 Sheets-Sheet 3 l 7% um T w Il. @amm a w n Q Gm f z F m5 4% 2 m a www A a 2 w l. a A ww a Woe/Vex yldfi iiitiltlihlstt Donovan C. Davis, Pasadena, Louis A. lille, Alhambra, and .iohn i?. Stoltz, Los Angeles, Calif., assignors to Giifiilan ilros., line., Los Angeles, Edif., a corporation of California File-rl dan. Z, 1957, Ser. No. 637,645 1li tlainis. (Cl. 329--l4l-ti) This invention relates to devices for producing directcurrent voltages havin.o amplitudes and algebraic signs which are functions of the difference between the frequency of an input signal and a predetermined frequency, and more particularly to discriminators having unambiguous responses.

The word unambiguous as used above refers to the operating characteristic of a frequency discriminator which has only one crossovcr, i.e. only one frequency at which its output is exactly equal to zero.

All present day frequency discriminators generally employ two bandpass lters and for this reason very often have several Crossovers in continuous wave (CW) operation as a result of any very slight misalignment of the filters. This effect is due to the fact that diiferencing is performed, not of small voltages, but of voltages which are large and nearly equal at frequencies far removed from the principal or designed crossover, i.e. the crossover at which the discriminator is designed to operate.

In pulse operation, these same discriminators have an even more unreliable response because additional crossovers are produced. Additional Crossovers are produced because of the scallop-shaped power spectrum of pulse modulated radio frequency (RF) signals. When the frequency spectrum of the pulse to be passed through a discriminator has a width which is of the same order as that of the pass band of the discriminator, an anomalous behavior is presented. Specifically, if the response of the discriminator to pulses of short duration is observed as a function of the center frequency of the pulse, several crossover points may be found.

Present day frequency discriminators therefore generally have several Crossovers, and for this reason, have several disadvantages and particularly when employed in scrvomechanisms. For example, when they are ernployed as components of automatic frequency control (AFC) circuits, this type of electronic servo may home on a crossover frequency other than the desired oscillation frequency of an associated oscillator.

The present invention overcomes these and other disadvantages of the prior art by providing a frequency discriininator which has only one crossover. This discriminator comprises a high-pass filter for attenuating au input signal substantially only when it has a frequency below a predetermined frequency, a low-pass filter for attenuating the input signal substantially only when it has a frequency above the predetermined frequency, and means for rectify/ing the outputs of each of the filters to produce a direct-current voltage having a predetermined polarity when the input signal has a frequency above a predetermined frequency, and to produce a direct-current voltage having a polarity opposite the predetermined polarity when the input signal has a frequency below the predetermined frequency. Although the output of each lter may be substantially constant over a wide range of frequencies, the discriminator has only one true crossover existing at its designed crossover.

By using a special feature of the invention, the sensi. tivity of this discriminator at its crossover may be increased to a relatively high value. This is done in accordance with the invention by terminating each of the filters in an impedance substantially greater than the cor- Patented tirolo. fi, i953 responding characteristic impedance of each. This may also be effected by using intermediate filters having impedances respectively greater than that of the source irnpedance to each. However, it must be remembered that the lter cannot be a bandpass filter of a rather limited bandwith if the discriminator is to be employed in pulse operation with pulses having relatively small time widths.

lt is therefore an object of the invention to provide a. frequency discriminator having a single and true designed crossover.

It is another object of the invention to provide a sensitive frequency discriminator capable of producing a reliable response over an extremely large band of frequencies.

The novel features which are believed. to be characteristie of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings. lt is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.

FIG. l is a schematic diagram of a frequency discriminator of the prior art;

FIG. 2 is a graph of a typical output characteristic of the discriminator shown in FlG. l;

FIG. 3 is a graph of the power spectrum of a pulse modulated RF signal;

FIGS. 4 and 5 are schematic diagrams of two alternative embodiments of the invention;

FlG. 6 is a detailed schematic diagram of another embodiment of the invention;

FIG. 7 is a graph of the frequency response of the discriminator shown in FIG. 6;

FIG. 8 is a detailed schematic diagram of still another embodiment of the invention; and

PEG. 9 is a graph of the output characteristic of the discriminator of FIG. 8 as compared to a conventional discriminator output characteristic.

in the drawing in FIG. l two bandpass filters iii and l2 are shown connected respectively to a diode output circuit ld including a diode 16 connected from filter l@ and a diode l5 connected from filter i2. It is to be noted that the diodes l5 and 1S are poled to be conducted in opposite directions. The output electrodes of each of the diodes lo and te are connected together and to a smoothins circuit 2t?.

in the operation of the discriminator shown in FlG. l, several crossovers may be encountered. The response of the discriminator might be that indicated at 2?, shown in HG. 2. Crossovers may occur frequencies f1, f2, fc, f3 and f4, where fc is the design crossover of the discriminator. This characteristic may be produced by slight misalignnrent of each of the filters ifi and 1.2,

In tron, additional Crossovers may be produced in pulse operation because during pulse operation the frequency spectrum of a pulse may be that as is indicated in FlG. 3 where fo is the fundamental carrier frequency of a pulse modulated wave. Y

The present invention overcomes both of these disadvantagcs of the prior art by providing a discriminator haring only one crossover. An e :rbodiment of the invention which may be employed to eliminate any crossovers except in the designed crossover is shown in FlG. 4. This discriminator comprises high and low pass lters 24 and 2o which are in turn connected to the same diode network ld and smoothing circuit 2i) shown in FIG. 4.

As shown in PEG. 5, if the phases of input signal to each of the filters 24 and 2a are reversed by transformer means 2S thereshown, the output may be passed through -inductor da.

Ational smoothing capacitors dd and diierent rectier and smoothing circuit means ffii and 32, the rectiiier means 3d including two i'ectitiers 34 and 36 connected respectively from filters 24 and 26, but poled in the same direction.

The discriminator shown in FiG. 6 is provided 'with output terminals 33 and du, tne voltages obtained at each being positive with respect to ground. When the discriminator is unbalanced in one direction, the voltage obtained at one of the terminals 38, do will be su stantially greater than he voltage obtained at the other. This discriminator was designed for a 130 megacycle crossover. Thesensitivity of t..e discriminator at the crossover frequency is a function of the input signal to noise ratio and the circuit element. For the 130 inegacycle per second input, a sensitivity of l miliivolts per megacycie at crossover was obtained. The output characteristic of the discriminator shown in FIG. 6 is shown in FiG. 7.

The discriminator of FlG. 6 includes a high pass filter 44 including serially connected capacitors d6 and ld and parallel connected inductors 5d and 52. A pair of parallel connected capacitors 34 and do are connected from inductor 52 to ground. A low pass lilter 53 is similarly provided with serially connected inductors 65) and 62 with a parallel connected capacitor 6d and a parallel connected A pair oi capacit-ors 68 and itl are then connected from inductor 66 to ground. Terminations are respectively provided for filters id and by resistors 72 and 74 respectively. A diode 76 is connected from the upper end of resistor 72 and a diode '7S is connected from thelower end of resistor 7d. Diode '7d is poled to be conductive in the direction to the left as viewed in llG. 6 and diode 73 is poled to be conductive in the direction to the right. Smoothing capacitors titi and 82 are connected respectively from the right hand side of diode 76 and the right hand side ot diode 78 to ground. Addi- 36 are respectively connected in parallel to capacitors dit and SZ. Smoothing resistors 33 and @d are respectively connected in parallel with smoothing capacitors dfi and S6.

A central conductor 92 is grounded at the right end thereof as viewed in FiG. 6. The discriminator of FiG. 6

provided with input terminals and 94', and output terminals 3% and dil, as stated previously.

A frequency discriminator t2 is shown in FIG. 8, which may be employed in an AFC circuit. A graph et a conventional discriminator output characteristic is indicated at C in FIG. 9. The output characteristic of the discriminator shown in EG. S is indicated at A and B in HG. 9. The characteristic A was found with a matched termination for each filter of, for example, 1GO ohm resistors. The characteristic i?. uvas found for mismatched terminations ot', tor example, ldd@ ohms, which is substantially greater than the G ohm characteristic inipedances of cach filter. it is to be noted that the output characteristic B does not create ambiguous crossovers although the discriminator sensitivity is increased at the design crossover fc, sensitivity being the slope of the output characteristics at frequency' fzfc.

in HG. 8 the high pass lter includes serially connected capacitors and 93 and a parallel connected inductor fritti. The lovv pass filter includes serially connected in- `ductors 92 and lit/i, parallel connected inductors loo and a parallel connected capacitor parallel connected c e ld?. and lid and lio are connected from inductor lud to grou connected cap-chers and ..22 inductor lille to ground.

A resistive termination including a resistor 124i is connected in parallel with the low pass iilter from a terminal of a diode E126 to ground. A resistive termination including a resist-or 12.3 is connected from a terminal of another diode lli to ground. Diode 26 is poled to be conductive in a direction to the right as viewed in HG. 8. Diode i3d is poled to be conductive in a direction to the nd. Three parallel are connected from left as viewed in HG. 8. he right hand terminals of each of the diodes lid and it? are connected to an output terminal E32 from which a capacitor lsfl is connected to ground.

Discriminator is provided with a ii'st input terminal ddii and a secc-nd grounded input terminal i353. Discrirninator 4Z is also provided not only with iirst output i3?. but also a grounded output terminal i3d.

in the circuit `ot PEG. 6, all values are given microfarads, niillihenries and ohms unless otherwise indicated. The circuit shown in FlG. 8 proved non-critical with a 50 rniilivolt input and a. .5 microsccond pulse. A 7 kilocycle error was suiiicient to operate an AFC servo motor. By taking appropriate but unnecessary design measures, this error could have been reduced to one kiiocycle.

in both the circuits of FEGS. 6 and 8 :rz-derived ter- Arninations are used, but they need not necessarily be used.

They may be designed to be Butterworth or rchebyschev types. in any case, one type is not preferred over another.

Other changes and modiiications iu the invention will, of course, suggest themselves to those skilled in the art and the invention is by no means limited to the specific embodiments shown since they are shown merely for purpose of illustration, but the invention and its true scope is only delined in the appended claims.

What is claimed is:

l. A frequency discriminator comprising: a high-pass filter for attenuating an input signal substantially only when it has a frequency below a predetermined frequency; a low-pass filter for attenuating said input signal substantially only when it has a frequency above said predetermined frequency, both of said lters being provided with mismatched terminations connected directly in parallel with each corresponding filter; and means for combining the outputs of both ot said lilters to produce a Vdirect-current voltage having a predetermined polarity when said input signal has a frequency above said predetermined frequency, aud a polarity opposite said predetermined polarity When said input signal has a frequency below said predetermined frequency, each of said mismatched terminations being connected to the input of said combining means.

2. A frequency discriminatori comprising: a high-pass filter for attentuating an input signal substantially only when it has a frequency below a predetermined frequency; a termination connected directly in parallel with said highpass filter, said termination having an impedance substantialiy greater than its characteristic impedance; a lowpass filter for attenuating said input signal substantially only when it has a frequency above said predetermined frequency; a termination connected directly in parallel with said low-pass filter, said termination having an irnpedance substantially greater than its characteristic irnpedance; and means for combining the outputs of both of said iilters to produce a direct-current voltage having a predetermined polarity when said input signal has a trequency above said predetermined frequency, and a polarity opposite said predetermined polarity when said input signal has a frequency below said predetermined irequency, said terminations beine connected to the input of said combining means.

3. A frequency descriminator comprising: a high-pass filter for attenuating an input signal substantially only when it has a frequency below a predetermined frequency; a restrictive termination connected directly in parallel with said high-pass tilter, said resistive termination having an impedance substantially greater than its characteristie impedance; a low-pass filter for attenuating said input signal substantially only when it has a frequency Y above said predetermined frequency; a resist-ive terminameans for combining the outputs ol both of said filters to produce a direct-current voltage having a predetermined polarity when said input signal has a frequency above said predetermined frequency, and a polarity opposits said predetermined polarity when said input signal has a frequency below said predetermined frequency, said resistive terminations being connected to the input of said combining means.

4. A frequency discriminator comprising: a high-pass lilter for attenuating an input signal substantially only when -it has a frequency below a predetermined frequency; a low-pass filter for attenuating said input signal substantially only when it has a frequency above said predetermined frequency; a resistive termination connected directly in parallel with each of said iilters, said resistive terminations being of an impedance substantially greater than the characteristic impedances of said iilters; a iirst rectifier connected from said high-pass lter and poled in one direction; a second rectifier connected from said low-pass filter and poled in the opposite direction, input electrodes of said rectitiers other than those connected to corresponding ones of said filters being connected together; and means connected from the junction of said rectiiiers for producing a substantially constant direct-current voltage representing the average value of the voltage appearing at the junction of said rectiers.

5. A frequency discriminator comprising: a high-pass filter for attenuating an input signal substantially only when it has a frequency below a predetermined frequency; a low-pass iilter for attenuating said input signal substantially only when it has a frequency above said predetermined frequency; a resistive termination connected directly in parallel with said filters, said resistive termination being of an impedance substantially greater than their characteristic impedances; a rectilier connected from the outputs of each of said filters, said rectiiiers being poled in the same direction and having input electrodes connected both to said filters and to said resistive termination; and means connected between output electrodes of said rectifiers for producing a substantially constant direct-current voltage having a predetermined polarity when said input signal has a frequency above said predetermined frequency, and a polarity opposite said predetermined polarity when the input signal has a frequency below said predetermined frequency.

6. A frequency discriminator comprising: a high-pass filter for attenuating an input signal substantially only when it has a frequency below a predetermined frequency; a low-pass filter for attenuating said input signal substantially only when it has a frequency above said predetermined frequency; a mismatched termination connected directly in parallel With each of said filters, said terminations bcing of a different order of magnitude than the characteristic impedances of said lilters; and means for combining the outputs of both of said filters to produce a direct-current voltage having a predetermined polarity when said input signal has a frequency above said predetermined frequency, and a polarity opposite said predetermined polarity when said input signal has a frequency below said predetermined frequency, said terminations being connected to the input of said combining means.

7. A frequency discriminator comprising: a high-pass iilter for attenuating an input signal substantially only when it has a frequency below a predetermined frequency; a low-pass ilter for attenuating said input signal substantially only when it has a frequency above said predetermined frequency; a mismatched termination connected directly in parallel with each of said filters, said terminations being of a higher order of magnitude than the characteristic impedances of said filters; and means for combining the outputs of both of said filters to produce a 'sasso direct-current voltage having a predetermined polarity when said input signal has a frequency above said predetermined frequency, and a polarity opposite said predetermined polarity when said input signal has a frequency below said predetermined frequenc, said terminations being connected to the input of said combining 3. A frequency discriminator comprising: a liigli-pass filter for attenuating an input signal substantially only when it has a frequency below a predetermined frequency; a low-pass iilter for attenuating said input signal substantially only when it has a frequency above said predetermined freouency; a mismatched termination connected directly in parallel with each of said filters, said terminations being about ten times as large as the characteristic impedances of said filters; and means for combining the outputs of boti of said filters to produce a direct-current voltage having a predetermined polarity 'when said input signal has a frequency above said predetermined frequency, and a polarity opposite said predetermined polarity when said input signal has a frequency below said predetermined frequency, eaeh of said mismatched terminations being connected to the input of said combining means.

9. A frequency discriminator comprising: a high-pass filter for attenuating an input signal substantially only when it has a frequency below a predetermined frequency; a low-pass filter for attenuating said input signal substantially only when it has a frequency above said predetermined frequency; a mismatched termination connected directly in parallel With each of said filters, said terminations being of a magnitude sufdciently large to produce a substantial increase in the rate of change of the output of said filters with respect to frequency at their crossover points over that which would be produced by terminating said filters in their characteristic impedances; and means for combining the outputs of both of said lters to produce a direct-current voltage having a predetermined polarity when said input signal llas a frequency above said predetermined frequency, and a polarity opposite said predetermined polarity when said input signal has a frequency below said predetermined frequency, each of said mismatched terminations being connected to the input of said combining means.

10. A frequency discriminator comprising: a high-pass filter for attenuating an input signal substantially only when it has a frequency below a predetermined frequency; a low-pass iilter for attenuating said input signal substantially only when it has a frequency above said predetermined frequency; a mismatched termination connected directly in parallel with each of said filters, said terminations being resistors having resistances of a higher order of magnitude than the characteristic impedances of said filters; and means for combining the outputs of both of said lters to produce a direct-current voltage having a predetermined polarity when said input signal has a frequency above said predetermined frequency, and a polarity opposite said predetermined polarity when said input signal has a frequency below said predetermined frequency, said mismatched terminations being connected to the input of said combining means.

References Cited in the tile of this patent UNITED STATES PATENTS 

1. A FREQUENCY DISCRIMINATOR COMPRISING: A HIGH-PASS FILTER FOR ATTENUATING AN INPUT SIGNAL SUBSTANTIALLY ONLY WHEN IT HAS A FREQUENCY BELOW A PREDETERMINED FREQUENCY; A LOW-PASS FILTER FOR ATTENUATING SAID INPUT SIGNAL SUBSTANTIALLY ONLY WHEN IT HAS A FREQUENCY ABOVE SAID PREDETERMINED FREQUENCY, BOTH OF SAID FILTERS BEING PROVIDED WITH MISMATCHED TERMINATIONS CONNECTED DIRECTLY IN PARALLEL WITH EACH CORRESPONDING FILTER; AND MEANS FOR COMBINING THE OUTPUTS OF BOTH OF SAID FILTERS TO PRODUCE A DIRECT-CURRENT VOLTAGE HAVING A PREDETERMINED POLARITY WHEN SAID INPUT SIGNAL HAS A FREQUENCY ABOVE SAID PRE- 